Embodiments of the present invention relate to integrated circuit design. More particularly, embodiments of the present invention provide a method and apparatus for providing S correction to a ramp voltage for controlling vertical deflection in a cathode ray tube that can be implemented on an integrated circuit without the need for external components.
In a cathode ray tube (CRT), such as a computer monitor or television set, an electron beam is swept horizontally and vertically across the screen. A linear ramp voltage is used to drive the vertical deflection yoke to produce the vertical component of the sweep. Because of non-linear behavior in a CRT""s vertical deflection system, a ramp with xe2x80x9cSxe2x80x9d correction is needed to correct its non-linearity. A typical equation describing a vertical ramp output with S correction is:
y=DC+a*x+b*x{circumflex over ( )}3
where a and b are constants, x is a linear ramp waveform, DC is an offset voltage and y is the S corrected ramp voltage.
In the prior art, analog circuits using multipliers and integrators typically generate the S correction. However, even though complex portions of such circuits may be implemented on integrated circuits, such S correction circuits still require external resistor and capacitor (RC) components.
These external components are required because the resistance and capacitance values and the required tolerances of those values are not commercially feasible to implement in integrated circuits.
Unfortunately, this requirement for external components has many undesirable consequences. Such external components consume many times the printed circuit board area of an integrated circuit, which results in larger circuit implementations, compared with a circuit design which could be implemented entirely in integrated circuits.
Additionally, the external components add significant expense to the implementation of the design (in comparison to a design without external components) in a number of areas:
First, external components contribute their individual component costs; second, external components require additional space, demanding a larger, more expensive printed circuit board; further, building the design requires a longer, more complex, and consequently more expensive printed circuit board assembly and test operation; and finally, external resistors and capacitors also increase manufacturing defect costs and field repair/warranty costs, since greater numbers of interconnects and greater numbers of components decrease reliability.
Clearly, there are numerous financial benefits to reducing or eliminating external components.
A further significant undesirable effect of using an RC network to apply S correction is that such an RC network is fundamentally sensitive to frequency. In many computer monitor designs, it is desirable to change the scan rate, in order to accommodate attachment to different computers, for example, or to change resolution modes.
Changing the scan rate frequency of such a prior art S correction circuit based on RC components will inexorably change the amount of the S correction. Such a change in amount of the S correction will visibly degrade the display image, for example moving picture elements at the beginning and end of a scan line out of alignment with the rest of the line. It is clear that this characteristic of the prior art is highly undesirable.
Therefore, it would be advantageous to provide a method and system providing S correction to a ramp voltage for controlling vertical deflection in a cathode ray tube that can be implemented on an integrated circuit without the need for external components. A further need exists for a circuit design where the amplitude of the S correction does not vary with sweep frequency. A still further need exists for a system and method of controlling the amplitude of the S correction internally to an integrated circuit.
The present invention provides a method and system for providing S correction to a ramp voltage for controlling vertical deflection in a cathode ray tube that can be implemented on an integrated circuit without the need for external components. Embodiments provide a circuit design in which the amplitude of the S correction does not vary with sweep frequency. Further embodiments provide for a system and method of controlling the amplitude of the S correction internally to an integrated circuit.
A method and apparatus for vertical S linearity correction with no external components is disclosed. A periodic waveform may be cubed to produce a second waveform. This second waveform may be sampled to obtain a peak value of the periodic waveform. The peak value may be multiplied by a constant to produce an offset value. The offset value may be subtracted from the second waveform to produce a third waveform. This third waveform may be added to the periodic ramp voltage. Consequently, the periodic waveform is modified by a cubic waveform.